1. Field of the Invention
The present invention relates to a device for protecting semiconductor material and to a method for protecting semiconductor material.
2. The Prior Art
High-purity semiconductor material is required for the production of solar cells or electronic components, such as for example storage elements or microprocessors. It is therefore desired to keep the concentrations of harmful contaminants as low as possible. It is often observed that semiconductor material which has already been produced to a high purity level is contaminated again during further processing to give the target products. For this reason, expensive cleaning steps are required again and again in order to restore the original purity. By way of example, foreign metal atoms which are incorporated in the crystal lattice of the semiconductor material interfere with the charge distribution and may reduce the ability of the subsequent component to function or may even lead to this component failing. Consequently, metallic contamination of the semiconductor material is to be particularly avoided. This is true in particular of silicon, which is the most frequently used semiconductor material in the electronics industry.
High-purity silicon is obtained, for example, by thermal decomposition of silicon compounds which are readily volatile and are therefore easy to purify by distillation methods, such as for example trichlorosilane.
To produce high-purity silicon using the most frequently employed Siemens method, a mixture of trichlorosilane and hydrogen is guided over thin silicon rods, which are heated by direct current passage to approximately 1100.degree. C., in a quartz reactor. The result is polycrystalline silicon in the form of rods with typical diameters of from 70 to 300 mm and lengths of from 500 to 2500 mm. The polycrystalline silicon is used to produce single crystals which are pulled from crucibles, strips and sheets or to produce polycrystalline solar-cell base material.
For the production of these products, it is necessary to melt solid silicon in crucibles. In order to achieve a high filling level in the crucibles and, in this way, to make the melting operation as efficient as possible, the abovementioned polycrystalline silicon rods have to be comminuted and then separated before being melted. Usually, this inevitably entails contamination to the surface of the semiconductor material, since the comminution is carried out using metallic crushing tools, such as jaw crushers or roll crushers, hammers or chisels on bases made from materials such as steel or plastic. Also, the subsequent separation operation usually takes place on screens made from metal or plastic. Therefore, during the comminution operation and the separation step, the silicon is contaminated by metals or carbon from the tools and the base. To eliminate this contamination, the fragments have to be subjected to complex and expensive surface cleaning, such as for example by etching with HF/HNO.sub.3, before being melted.
Therefore, silicon bases and tools made from silicon or with silicon coatings are used to reduce the contamination during comminution. Screens made from silicon or silicon-coated screens also form part of the prior art for the separation operation. However, these have the drawback that they are damaged or destroyed by the transmission of forces during the comminution operation (striking with hammers) or in the separation operation, and have to be replaced. On average, a base is able to take approximately 10 to 15 batches (corresponding to approximately 10 to 15 t) both during crushing and during screening. Then, broken pieces (approx. 30%) have to be replaced, so that fragments of the base do not enter the material which is to be sold.
Furthermore, these silicon bases have to be disposed of, entailing further costs, since this material is cracked or has been comminuted to undesirable fragment sizes, and therefore can no longer be sold. The production of such a base requires additional deposition of silicon, mechanical operations involved in the production of the shaped parts and complex cleaning thereof, for example by etching with HF/HNO.sub.3.